Low pressure method for injection molding a plastic article

ABSTRACT

A method for injecting a plastic material into a mold cavity to precisely fill the mold cavity under low pressure and to avoid packing the mold cavity with plastic material. The plastic material is injected into the mold cavity at a rate sufficient to maintain an unbroken melt front and to avoid any spraying or splashing of the plastic material within the mold cavity. After an initial unbroken melt front is established, the rate at which the material is injected is increased in proportion to the size of the melt front. After a predetermined amount of the material has entered the mold cavity, the material is injected based upon a rate-dominated control algorithm which maintains the unbroken melt front. Once the mold cavity has reached a pre-determined level of fill, the rate-dominated control algorithm is changed to a pressure-dominated control algorithm to prevent overrunning the end point of the mold process and to eliminate splashing and associated backflow problems. The injection pressure is monitored and controlled until the mold cavity has been precisely filled, at which time the injection pressure is maintained until the plastic article has hardened within the mold cavity. By filling the mold cavity exactly, the prior art problems associated with packing, such as warping and flash, are eliminated in the present invention and a more uniform product is produced from less material.

RELATED APPLICATIONS

This present application is a continuation of U.S. patent applicationNo. 08/492,232 filed on Jun. 19, 1995 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a method for molding a plastic article and,more particularly, to a method for molding a plastic article which usesincremental monitoring of the injection process without having tooverpack the mold unit with plastic material.

Injection molding machines generally include a two-section mold unitwherein one of the mold sections is stationary and includes an end gateopening. The opening allows the injection of thermoplastic material intoa cavity formed by the mold unit. The other mold section is generallymovable between an open position away from the stationary mold section,and a closed position wherein the two mold sections are in sealedcontact engagement to form the mold cavity.

Once the mold cavity has been formed, a reciprocating screw or similarinjection device is used to inject a plastic material into the moldcavity where the material hardens with time. During injection of theplastic material, the mold unit is typically cooled. The cooling causesthe plastic material to harden faster, so that the mold unit can be usedto mold a greater number of plastic articles than if the mold unit werenot cooled. As the plastic material is injected into the mold cavity,the leading edge of the plastic material forms a melt front whichspreads across the mold cavity as the mold cavity fills. The plasticmaterial, however, is typically injected into the mold cavity veryquickly under high pressure to prevent the melt front from hardening tooquickly and blocking the mold cavity. This high pressure injectionbreaks the smooth melt front and causes an erratic spray of plasticmaterial into the mold cavity. Accordingly, instead of an unbrokensymmetrical melt front evenly filling the mold cavity, the plasticmaterial is sprayed throughout the mold cavity, thereby causing anasymmetrical filling of the cavity. This asymmetrical filling has theundesirable effect of preventing the plastic from filling every portionof the mold cavity. Additionally, as the plastic material is sprayedinto the mold cavity, portions of the mold cavity are filled beforeothers, leading to irregular hardening of the plastic material. Afurther problem with typical high pressure injection mold fillingtechniques is pressure needed to overcome such erratic filling of themold cavity. Just as it is difficult to fill a glass of water completelywith a high pressure hose, so too is it difficult to completely fill amold cavity with high pressure injection techniques. As the cavity isfilled, the plastic material begins to backflow, causing turbulence andrequiring greater clamp pressures and injection pressures.

To overcome the asymmetrical filling and backflow associated with priorart processes, and to assure that the mold cavity becomes completelyfilled with plastic material, a volume of plastic material greater thanthe volume of the mold cavity is injected into the mold cavity andmaintained under high pressure until the plastic begins to harden. Theinjection of an excess amount of material into the mold cavity requiresa sharp pressure increase or "spike" to pack the material into the moldcavity. While "packing" the mold cavity with an excess of plasticmaterial completely fills the mold cavity during the molding process,the pressure spike causes enormous outward pressure on the moldsections, leading to an increased amount of wear on the mold sectionsand clamping apparatus. Despite constructing mold units of steel tohandle the pressure spikes associated with prior art molding processes,these mold units often wear prematively, requiring the mold units to bereturned to the manufacturers for repair. Depending on the severity ofthe wear and the repair schedule of the manufacturers, worn mold unitscan be out of service for several weeks. The loss of productionassociated with the repair of cracked mold units is generally verycostly.

The increased outward pressure on the mold sections may even lead tomovement of the mold sections away from one another. When the moldsections move apart, plastic material seeps into the parting linebetween the mold sections. This "flash" is not only aestheticallyundesirable on a finished plastic part, but also leads to a waste ofmaterial and creates an uneven parting line between the mold sections.As the clamping apparatus presses the mold sections together against theplastic hardening between the parting line, the parting line becomesdeformed and uneven, thereby making flash even more likely uponsubsequent moldings.

Additional problems associated with prior art high pressure moldingtechniques are the increased amount of material needed to be placed intothe mold cavity, as well as the pressure gradients created throughoutthe molded part due to the increased amount of material being packedinto a finite volume mold cavity. In some cases, the pressure gradientis so large that it leads to warpage of the finished plastic article. Ifthe warpage is great enough, the plastic article may not fit withindesign tolerances.

While it would be desirable to fill the mold cavity slowly throughoutthe molding process, to avoid asymmetrical filling and backflow, it isdifficult to inject the plastic material slowly without uneven curingand blockage within the cavity. If the mold cavity is filled too slowly,the first portion of the plastic material to be injected into the moldcavity begins to harden as the rest of the plastic material is stillbeing injected. This premature hardening leads to clogging of the moldcavity during the filling process. Such clogging is particularlyundesirable since it requires stopping a molding run and discarding thepartially hardened material.

The difficulties encountered in the prior art discussed hereinabove aresubstantially eliminated by the present invention.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for moldingan injected plastic article which requires less clamping force thantraditional plastic injection mold units.

Another object of the present invention is to provide a method forproducing an injected plastic article without pressure gradients orwarpage in the plastic article.

Yet another object of the present invention is to provide a method formolding an injected plastic article which reduces material costs in themolding process.

Still another object of the present invention is to provide a method formolding an injected plastic article which produces more uniformreproducible plastic articles.

Another object of the present invention is to provide a method forprecisely filling a mold cavity of mold unit.

These and other objects of the invention will become apparent uponreference to the following specification, drawings, and claims.

By the present invention, it is proposed to overcome the difficultiesencountered heretofore. To this end, a method for directing andcontrolling the operation of an injection molding machine in a mannerwhich maintains an unbroken melt front is provided. A first mold sectionand a second mold section are provided, wherein the first mold sectionand the second mold section form a mold cavity for molding a plasticarticle when the second mold section is pressed into sealed engagementwith the first mold section. To begin the molding process, the secondmold section is pressed into sealed engagement with the first moldsection to form the mold cavity. An injection device is provided inoperable engagement with the mold cavity and a plastic material isinjected into the mold cavity with the injection device. The plasticmaterial is injected at a rate which deposits the plastic material intothe mold cavity with an unbroken melt front along a leading edge of theplastic material. The rate at which the plastic material is injectedinto the mold cavity is increased while the unbroken melt front alongthe leading edge of the plastic material is maintained. The pressure atwhich the plastic material is injected into the mold cavity isthereafter decreased before the mold cavity is completely filled withplastic material. This pressure reduction significantly reduces theamount of clamp pressure needed to keep mold halves together, but doesnot prevent the mold cavity from becoming completely filled.

In the preferred embodiment of the present invention, a variable volume,pressure compensated hydraulic pump is used to control the injection ofthe plastic material, while a central processing unit is operablyconnected to both the variable volume pressure compensated hydraulicpump and a shot size transducer to monitor the plastic material enteringthe mold cavity. Preferably, the plastic material within is placedwithin an injection device. The central processing unit triggers theinjection device to inject the plastic material into the mold cavitywith the injection being dictated by an initial rate-dominated controlalgorithm. Via the shot size transducer, the central processing unitmonitors the amount of plastic material entering the mold cavity inincrements of a predetermined size. The central processing unitincreases the injection rate from the initial rate-dominated controlalgorithm to a faster, rate-dominated control algorithm. This increasein injection rate comes after approximately ten percent of theincrements of plastic material have entered the mold cavity.

The central processing unit continues to monitor the number ofincrements of plastic material entering the mold cavity untilapproximately seventy percent of the increments have been injected intothe mold cavity. At this time, the central processing unit triggers theinjection device to inject the plastic material based upon apressure-dominated control algorithm. The pressure-dominated controlalgorithm injects the plastic material into the mold cavity more slowlythan the faster, rate-dominated control algorithm. The pressurereduction eliminates turbulence and allows the mold cavity to completelyfill with plastic material, thereby eliminating the need for overpackingthe mold cavity with plastic material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing the method of the present invention;

FIG. 2 is a side elevation of the molding apparatus of the presentinvention;

FIGS. 3a-c are top plan views of a prior art mold cavity in partialcross-section, showing the progress of plastic material through the moldcavity;

FIGS. 4a-c are top plan views of the mold cavity of the presentinvention in partial cross-section, showing the progress of plasticmaterial through the mold cavity;

FIG. 5 is a graphic representation of the injection rate during themolding process of the present invention; and

FIG. 6 is a graphic representation of the injection pressure during themolding process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the figures, a mold apparatus 10 is shown including a mold unit 12, amold frame 14, and a hydraulic piston 16 (FIG. 2). The mold frame 14consists of four cylindrical support bars 18 which are secured to afirst plate 20 and a second plate 22 by means of nuts 24. A carriage 26is slidably attached to the support bars 18 through bores which passthrough the corners of the carriage 26. Support blocks 28 are providedwith bores and mounted to the corners of the carriage 26 to add extrasupport to the carriage 26 as it slides along the support bars 18. Aplaten 30 is secured to the carriage 26 to evenly distribute force overthe carriage 26. Secured to the platen 30 is a piston ram 32. The pistonram 32 passes through a bore in the center of the second plate 22 and isconnected to a hydraulic cylinder 34. The hydraulic cylinder 34 andpiston ram 32 make up the hydraulic piston 16.

Secured to the side of the carriage 26 opposite the platen 30 is a firstmold section 36 which fits into sealed engagement with a second moldsection 38 to form a mold cavity 40 (FIG. 2). The dimensions of the moldcavity 40 created by the first mold section 36 and second mold section38 define the shape of the plastic article to be molded. The second moldsection 38 is secured to the first plate 20 which, in turn, is securedto the mold frame support bars 18 of the mold frame 14. The first plate20 is secured on a first pair of support legs 42 and the second plate 22is secured on a second pair of support legs 44. The support legs 42 and44 position the mold unit 12 at the proper height for the moldingprocess.

Provided in the first plate 20 is an inlet sprue 46 which connects to arunner 48 provided in the first mold section 36 (FIG. 2). Because thepresent method uses slower injection than prior art methods, the size ofthe sprue 46 is preferably larger than sprues of the prior art. Thelarger sprue 46 allows a greater amount of plastic material 82 to enterthe mold cavity 40 through the sprue 46. Unlike heated sprues of theprior art, the sprue 46 of the present invention is not required to beheated due to its increased size. The runner 48 allows plastic materialto pass from the sprue 46 into the mold cavity 40.

Provided for operable engagement with the inlet sprue 46 is an injectionassembly 50 which prepares and injects the plastic material 82 into themold cavity 40 (FIG. 2). The injection assembly 50 is provided with aninjection barrel 52 supported by a main frame 54. A nozzle unit 56 ismounted at one end of the injection barrel 52 and a hopper 58 is mountedto the top of the injection barrel 52. Positioned coaxially within theinjection barrel 52 is a reciprocating screw 60 with attached flights(not shown) for plasticizing and moving the plastic material 82 towardthe injection barrel 52 and nozzle unit 56. A variable volume, pressurecompensated hydraulic pump 62 is secured to the injection barrel 52 andoperates to move the injection screw 60 relative to the injection barrel52 during the injection molding process. The injection assembly 50 isprovided with a large rotational motor 72 which turns the injectionscrew 60 to plasticize the plastic injection material 82 beforeinjection.

The nozzle unit 56 is movable into and out of operative association withthe sprue 46 upon a reciprocal movement of a carriage 64 relative to themain frame 54. This reciprocal movement is responsive to the operationof a double acting cylinder 66 pivotally interconnected between thecarriage 64 and the main frame 54. The operation of the double actingcylinder 66 is controlled by a linear actuator 68 mounted on themainframe 54 for coacting engagement with a limit member 70 on thecarriage 64.

The injection assembly 50 is operably coupled to a central processingunit 74 which monitors the progress of the injection process and feedsback responsive information regarding this progress to the injectionassembly 50 (FIG. 2). In the preferred embodiment the central processingunit 74 is a personal computer, but the central processing unit 74 may,of course, be any system capable of monitoring the progress of a plasticinjection run and controlling the injection in response thereto. Tomonitor the rate of plastic material injection, a shot size transducer76 is coupled to the injection screw 60 to monitor the distance that theinjection screw 60 travels. The shot size transducer 76 is a linearactuator which correlates movement of the injection screw 60 to apredetermined amount of the plastic material injected into the moldcavity 40. As each incremental amount of plastic material within theinjection assembly 50 is injected into the mold cavity 40, the shot sizetransducer 76 monitors the associated incremental movement of theinjection screw 60. Preferably the shot size transducer 76 measuresincrements of plastic material of one gram or less, which translatesinto approximately four thousand increments for a typical moldingoperation.

To monitor the pressure at which plastic material is injected into themold cavity 40, a pressure monitor 78 is operably coupled to thehydraulic pump 62 (FIG. 2). The pressure monitor 78 is also coupled tothe central processing unit 74. The central processing unit 74 isoperably coupled to an injection control 80 which is, in turn, coupledto the hydraulic pump 62 to manipulate the injection of the plasticmaterial 82.

To begin the molding process, the first mold section 36 is pressed intosealed engagement with the second mold section 38 by the hydraulicpiston 16 (FIG. 2). Once the mold sections 36 and 38 have been pressedinto sealed engagement to form the mold cavity 40, the double actingcylinder 66 moves the nozzle unit 56 of the injection assembly 50 intoengagement with the injection sprue 46. The plastic material 82 isdeposited into the hopper 58 and the reciprocating screw 60 is revolvedwith the motor 72 to plasticize the plastic material 82 and to transferthe plastic material 82 from the hopper 58 into the injection barrel 52.The reciprocating screw 60 is operably coupled to a variable backpressure relief valve which is well known in the art (not shown).Commands sent to the relief valve by the central processing unit 74during the plasticization stage determine the condition of the plasticmaterial 82, increment by increment before it is injected into themolding cavity 40.

When the injection barrel 52 is filled with properly plasticized plasticmaterial 82, the hydraulic pump 62 pushes the reciprocating screw 60toward the nozzle unit 56. The plasticized plastic material 82 isthereby transferred from the injection barrel 52 into the sprue 46 andrunner 48. From the runner 48, the plastic material 82 enters the moldcavity 40 (FIGS. 2 and 4a). The first mold section 36 and second moldsection 38 are preferably maintained at a constant temperature toprevent the plastic material 82 from prematurely hardening on the wallsof the mold cavity 40. This is in direct contravention of prior artmethods which teach significantly cooling the mold section to speedhardening and thereby increase production rates.

Initial injection of the plastic material 82 into the mold cavity 40 isat a pre-determined rate which is greatly reduced from prior artmethods. Prior art methods splatter the plastic material 82 throughoutthe mold cavity 40 which leads to undesirable unfilled voids. In thepresent method the predetermined rate is sufficiently slow to allow theplastic material 82 to enter the mold cavity 40 with an unbroken meltfront 84 as shown in FIG. 4a. This pre-determined initial injection ratemay be determined by trial and error. If the initial injection rate istoo high, the plastic material 82 entering the mold cavity 40 splattersas shown in FIG. 3a. This splattering is a result of high pressureinjection used in the prior art, and results in asymmetrical filling ofthe mold cavity as shown in FIGS. 3b and 3c. The asymmetrical fillingleads to a mold cavity 40 with various unfilled portions 86 as shown inFIGS. 3b-c. Since these unfilled portions 86 are scattered randomlyabout the mold cavity 40, it is not possible to fill these portions 86merely by adding more plastic material 82 at the previous rate.

To overcome this partial nonfilling problem, prior art processes "pack"an excess amount of plastic material 82 into the mold cavity 40 underextremely high pressure. This high pressure causes excessive strain onthe clamping unit and creates finished parts that have hardened withinternal pressure gradients. The pressure gradients may lead to warpageof the finished product and may, in extreme circumstances, cause theplastic article to be discarded as unusable. The high pressure alsoleads to "flashing" whereby material seeps between the mold sectionsduring the molding process. This "flash," apparent on some prior artproducts, is caused when the extreme pressure developed during thepacking phase forces the mold sections slightly apart enough so that theplastic material can seep along the parting line. As the plastichardens, the finished plastic article is left with a ridge of hardenedplastic along the parting line of the mold unit. In addition to beingundesirable and wasteful, over time this flash can lead to prematurewear of the mold sections as the clamp attempts to push the moldsections together against the hardened plastic.

In the preferred embodiment of the present method, movement of the moldsections 36 and 38 relative to one another is monitored by a humanoperator (not shown) and corrections manually inputed. By observing themold sections 36 and 38 over a series of molding runs, the operator canprogram the central processing unit 74 to decrease the injectionpressure enough to just fill the mold cavity 40 without causing flash orundue pressure within the mold cavity 40. Alternatively, software may bewritten so that the central processing unit 74 automatically "observes"movement of the mold sections 36 and 38 via a linear transducer (notshown) and adjusts the final injection pressure until the desiredinjection pressure is achieved.

As shown in FIG. 4a, as the plastic material 82 enters the mold cavity40, an initial unbroken melt front 84 is formed within the mold cavity40 (FIGS. 2 and 4a). To determine the initial rate of injection, themovement of the shot size transducer 76 is correlated against time ofinjection by the central processing unit 74. Since the shot sizetransducer 76 is correlated to monitor the injection of the plasticmaterial 82 into the mold cavity 40 in increments of one gram or less,the shot size transducer 76 detects incremental movements of thereciprocating screw 60 and transfers this information to the centralprocessing unit 74 for each increment of plastic material that isinjected into the mold cavity 40.

During the injection process, it is advantageous to maintain the meltfront intact to avoid splattering and asymmetrical filling of the mold.As more material is injected into the mold cavity 40, the melt front 84grows larger as shown in FIG. 4b. As the melt front 84 gains a largerand larger surface area, the plastic material 82 may be injected intothe mold cavity 40 at a higher rate without causing the melt front 84 tobreak. The increased injection rate will not increase the actualpressure along any particular point of the melt front 84 as long as theinjection rate is increased no faster than the rate at which the surfacearea of the melt front 84 grows.

As shown in FIG. 1 the plastic material 82 is injected into the moldcavity 40 at an initial pre-determined injection rate until the centralprocessing unit 74 receives information from the shot size transducer 76that ten percent of the one gram increments of the plastic material 82have been injected into the mold cavity 40 (FIGS. 1, 2, and 4a). Onceten percent of the increments have been injected, the central processingunit 74 triggers the injection control 80 to increase the injection ratein proportion to the amount of plastic material 82 which has beeninjected into the mold cavity 40. Because the surface area of the meltfront 84 at any given time is roughly proportionate to the amount ofplastic material 82 which has been injected into the mold cavity 40,increasing the injection rate in proportion to the amount of plasticmaterial 82 injected into the mold cavity 40 results in an increase inthe rate of injection which is proportionate to the surface area of themelt front 84. It should be noted that the increase may be initiatedafter between about two percent and fifty percent of the plasticmaterial 82 has been injected into the mold cavity 40.

The injection rate is increased in proportion to the surface area of themelt front 84 until twenty percent of the increments of the plasticmaterial 82 have been injected into the mold cavity 40 (FIGS. 1, 2, and4). Increasing the injection in proportion to the surface area of themelt front indefinitely, however, would lead to an undesirably highinjection rate which would be too difficult to slow. Accordingly, aftertwenty percent of the increments of the plastic material 82 have beeninjected into the mold cavity 40, the injection is continued using arate-dominated control algorithm to maintain the melt front 84 at adesirable injection rate. As shown in FIG. 5, in the preferredembodiment of the present process, the injection rate is increased untilapproximately twenty percent of the mold cavity is filled, at which timethe injection rate is steadily reduced until the mold cavity 40 isapproximately seventy percent filled. The rate-dominated controlalgorithm may be initiated any time after the increase in injection ratehas begun and is preferably discontinued before ninety-five percent ofthe mold cavity 40 is filled. The exact rate-dominated algorithmcontrolling the rate at which the plastic material 82 is injected intothe mold cavity 40 is not of critical importance, and may be customizedto provide desirable filling characteristics based upon a mold flowanalysis of the particular mold cavity 40. It is desirable, however, asshown in FIG. 5, to begin reducing the rate of injection well before themold cavity 40 has become ninety-five percent filled, to avoidundesirable backflow and splashing as the mold cavity 40 is filled tocapacity.

At any time during the injection of the increments of the plasticmaterial 82 into the mold cavity 40, where the pressure required exceedsthe requested pressure, the central processing unit 74 can trigger theinjection control 80 to inject the plastic material 82 based upon apressure-dominated control algorithm, rather than the rate-dominatedcontrol algorithm used to inject the majority of the plastic material82. The pressure-dominated control algorithm reduces the pressure atwhich the plastic material 82 is injected into the mold cavity 40 toprevent the backflow and turbulence associated with the prior artmethods. Because the prior art makes no correlations between injectionpressure and the speed at which the plastic material 82 fills the moldcavity 40, there is no way to anticipate the endpoint for the filling ofthe mold cavity 40. Therefore, in prior art techniques, there is no wayto accurately reduce the pressure to prevent turbulence and backflow.Indeed, the prior art methods actually increase the ending pressure toforce an overflow amount of plastic material into the mold cavity.

Like the rate-dominated control algorithm, the pressure-dominatedcontrol algorithm of the present method can be determined based uponmold flow characteristics of the particular mold cavity 40 and initiatedany time after the rate-dominated control algorithm has been initiated,but is preferably initiated after fifty percent of the increments of theplastic material 82 have been injected into the mold cavity 40. Thecentral processing unit 74 receives input from the pressure monitor 78to maintain a pressure on the plastic material 82. Preferably thispressure is sufficient to completely fill the mold cavity 40, withoutcausing the plastic material 82 to splash, or the melt front 84 to breakbefore the mold cavity 40 is filled (FIGS. 2 and 4c).

As shown in FIG. 6, the pressure is manipulated to prevent anyaberrations in the melt front 84 during the last portion of theinjection process. As shown in FIG. 4c, the present method evenly fillsthe entire mold 40, thereby eliminating the need for prior art "packing"techniques required to fill unfilled portions 86 produced by excessiveinjection rates and broken melt fronts (FIG. 3c). The central processingunit 74 triggers the injection control 80 to cease injection of plasticmaterial 82 into the mold cavity 40 at the proper point to fill the moldcavity 40 exactly. By providing the mold cavity 40 with precisely theamount of plastic material 82 that is needed to fill the mold cavity 40,there is a material savings over prior art packing processes. Thecentral processing unit 74 preferably coordinates the size of the shotwith the rate-dominated injection and the pressure-dominated injectionto achieve the proper balance between rapid and accurate fillings of themold cavity 40.

Near the end of the molding cycle in prior art processes, there is apressure spike when the plastic material reaches the end of the moldcavity and begins to backflow. This pressure is returned back to theinjection gate and requires large amounts of clamping force to maintainthe mold sections together against this end pressure. As this pressureis spread throughout the plastic material during the hardening process,the finished part often has molded-in stress which may weaken ordiscolor the part. Because packing is eliminated in the presentinvention, less clamping force is required to maintain the mold cavity40 during the mold process and molded in stress is reduced. Whereas twoand one-half to three tons of clamp pressure per square inch may berequired to hold the mold sections together in a prior art moldingoperation, one-half to one ton of clamp pressure per square inch may beused in molding a similar part by the present method.

Since the present method eliminates the pressure spike at the end of themolding process, less strength is required of the mold sections 36 and38. Accordingly, lighter weight aluminum may be used to make the moldsections. Additionally, prior art molding machines with pressure relatedfractures of their steel mold sections may be retrofitted with thealuminum mold sections 36 and 38 of the present invention. This allowsthe molding machine to be quickly returned to operation using thepresent inventive method. The reduction in clamping force also allowsthe use of cheaper, lighter, and smaller clamps. Furthermore, becausethe stress on the mold sections 36 and 38 is reduced, there is lesschance of failure of the mold sections of the present invention andtherefore less chance of the accompanying costly downtime.

Once the mold cavity 40 is filled, the plastic material 82 is allowed toharden within the mold cavity 40 (FIGS. 2 and 4c). After the plasticmaterial 82 has hardened, the hydraulic piston 16 is actuated to movethe movable mold section 36 away from the stationary mold section 38 sothat the hardened plastic material 82 may be removed from the moldapparatus 10. Because the present invention eliminates the excessiveprior art pressures during the hardening phase, the resulting plasticarticle is free of pressure gradients and warpage. This allows thepresent invention to produce a more uniform, reproducible plasticarticle.

In the preferred embodiment, the rate dominated control and thepressure-dominated control algorithms work together throughout theentire shot where the rate-dominated control algorithm is the leadingfactor and the pressure-dominated control algorithm is used as an upperlimit. At the point during the injection cycle where the pressurerequired by the rate-dominated control algorithm reaches the upper limitof the pressure-dominated control algorithm, the pressure dominatedcontrol algorithm takes over to finish the injection stroke.Accordingly, the algorithm of the preferred embodiment would be asfollows:

    ______________________________________                                        Actual =     Requested                                                                              While Actual                                                                              Is less than                                                                          Injection                           Injection    Injection      Pressure      Pressure                            Rate         Rate                         Limit                                                                         Set Point                           else                                                                          Actual   Decreases                                                                              Prevent Actual                                                                              From    Injection                             Injection                                                                              to               Pressure                                                                            Exceeding                                                                             Pressure                              Rate                                    Limit                                                                         Set Point                             ______________________________________                                    

The foregoing description and drawings merely explain and illustrate theinvention. The invention is not limited thereto, except insofar as theclaims are so limited, as those skilled in the art who have a disclosurebefore them will be able to make modifications and variations thereinwithout departing from the scope of the invention. For example, it isanticipated to be within the scope of the invention that fabric may bemaintained against one or both of the mold sections 36 and 38 during themolding process to provide a finished plastic article with a molded-infabric covering. The large sprue 46 and slow injection of the presentinvention decrease the incidences of wrinkling or tearing the fabricover prior art methods. It is also anticipated that the aforesaid pointsfor beginning the rate-dominated control algorithm and switching to thepressure-dominated control algorithm may be manipulated to exploitspecific characteristics of a particular mold cavity mold flow analysis.

What is claimed is:
 1. A method for directing and controlling the flowof plastic material into a mold cavity of a mold unit in a manner whichmaintains an unbroken melt front on the injected plastic materialthroughout the molding process, the steps of the method comprising:(a)providing a first mold section; (b) providing a second mold sectionwhich, when pressed into sealed engagement with said first mold section,forms a mold cavity for molding a plastic article; (c) pressing saidsecond mold section into sealed engagement with said first mold sectionto form said mold cavity; (d) providing an injection device in operablecommunication with said mold cavity; (e) injecting a plastic materialinto said mold cavity with said injection device at a rate sufficient toproduce an unbroken melt front along a leading edge of said plasticmaterial; (f) increasing the rate at which said plastic material isinjected into said mold cavity, while maintaining said unbroken meltfront along said leading edge of said plastic material; and (g) afterincreasing the rate of injection, but before said mold cavity iscompletely filled with said plastic material, decreasing the rate atwhich said plastic material is injected into said mold cavity whilemaintaining said unbroken melt front along said leading edge of saidplastic material.
 2. The method of claim 1, wherein said step ofincreasing the rate of plastic material injection includes increasingthe rate of injection in proportion to the size of the melt front. 3.The method of claim 1, wherein said step of increasing the rate ofplastic material injection is instigated only after a predeterminedportion of said plastic material has entered said mold cavity.
 4. Themethod of claim 1, wherein said step of decreasing the pressure at whichsaid plastic material is injected into said mold cavity is instigatedonly after a predetermined portion of said plastic material has beeninjected into said mold cavity.
 5. The method of claim 1, furthercomprising monitoring the amount of said plastic material which isinjected into said mold cavity with a linear transducer which isprovided on said injection device.
 6. The method of claim 5, furthercomprising controlling said step of injecting said plastic material,said step of increasing the rate at which said plastic material isinjected, and said step of decreasing the pressure at which said plasticmaterial is injected, with a central processing unit which is operablycoupled to said linear transducer.
 7. The method of claim 1, whereinsaid step of increasing the rate of plastic material injection comprisesinjecting said plastic material into said mold cavity using arate-dominated control algorithm.
 8. The method of claim 1, wherein saidstep of decreasing the pressure of plastic material injection comprisesinjecting said plastic material into said mold cavity using apressure-dominated control algorithm.
 9. A method for directing andcontrolling the flow of plastic material into a mold cavity of a moldunit in a manner which maintains an unbroken melt front on the injectedplastic material throughout the molding process, the steps of the methodcomprising:(a) providing a first mold section; (b) providing a secondmold section which, when pressed into sealed engagement with said firstmold section, forms a mold cavity for molding a plastic article; (c)pressing said second mold section into sealed engagement with said firstmold section to form said mold cavity; (d) providing an injection devicein operable communication with said mold cavity; (e) providing saidinjection device with a shot of plastic material sufficient to fill saidmold cavity; (f) dividing said injection shot into a plurality ofsubstantially equal increments; (g) injecting said injection shot intosaid mold cavity at an initial injection rate sufficient to create anunbroken melt front having a face, said melt front being created along aleading edge of said plastic material within said mold cavity; (h)increasing said initial injection rate after a first predeterminednumber of increments of plastic material have been injected into saidmold cavity, wherein said initial injection rate is increasedproportionately to the size of said face of said melt front; (i) afterincreasing the initial injection rate, decreasing said rate at whichsaid plastic material is injected into said mold cavity after a secondpredetermined number of increments of plastic material have beeninjected into said mold cavity while maintaining said unbroken meltfront along said leading edge of said plastic material, wherein saidsecond number of increments is larger than said first number ofincrements; (j) filling said mold cavity with said plastic material; (k)allowing said plastic material to at least partially harden within saidmold cavity; and (l) removing said plastic material from said moldcavity.
 10. The method of claim 9, wherein said increments of saidplastic material are less than about two grams each.
 11. The method ofclaim 9, wherein said step of increasing said initial injection rate isperformed only after at least two percent of said increments of plasticmaterial have been injected into said mold cavity.
 12. The method ofclaim 9, wherein said step of decreasing the pressure at which theplastic material is injected into said mold cavity is performed onlyafter at least fifty percent of said increments of plastic material havebeen injected into said mold cavity.
 13. The method of claim 9, furthercomprising providing said injection device with a linear actuatorcapable of monitoring the number of increments of plastic material whichhave been injected into said mold cavity.
 14. The method of claim 13,further comprising coupling said linear actuator and said injectiondevice to a central processing unit capable of increasing said initialinjection rate and decreasing the pressure at which said plasticmaterial is injected into said mold cavity after a predetermined numberof said increments of plastic material have been injected into said moldcavity.
 15. The method of claim 9, wherein said step of increasing therate of plastic material injection comprises injecting said plasticmaterial into said mold cavity using a rate-dominated control algorithm.16. The method of claim 9, wherein said step of decreasing the pressureof plastic material injection comprises injecting said plastic materialinto said mold cavity using a pressure-dominated control algorithm. 17.A method for directing and controlling the flow of plastic material intoa mold cavity of a mold unit in a manner which maintains an unbrokenmelt front on the injected plastic material throughout the moldingprocess, the steps of the method comprising:(a) providing a first moldsection; (b) providing a second mold section which, when pressed intosealed engagement with said first mold section, forms a mold cavity formolding a plastic article; (c) pressing said second mold section intosealed engagement with said first mold section to form said mold cavity;(d) providing an injection device in operable communication with saidmold cavity; (e) providing a hydraulic pump in operable communicationwith said injection device; (f) coupling a shot size transducer to saidhydraulic pump; (g) coupling a central processing unit to said hydraulicpump, and to said shot size transducer, wherein said central processingunit is capable of measuring incremental movements of said hydraulicpump; (h) providing said hydraulic pump with a plastic material; (i)dividing said injection shot into a plurality of substantially equalincrements of plastic material; (j) injecting a first predeterminednumber of increments of said plastic material into said mold cavity withsaid hydraulic pump at an initial injection rate sufficient to create anunbroken melt front along a leading edge of said plastic material, saidunbroken melt front having a face; (k) increasing said initial injectionrate after said first predetermined number of increments of plasticmaterial have been injected into said mold cavity, wherein said initialinjection rate is increased in proportion to the size of said face ofsaid melt front until a second predetermined number of increments ofplastic material have been injected into said mold cavity; (l) injectingsaid plastic material into said mold cavity using a rate-dominatedcontrol algorithm after said second predetermined number of incrementsof plastic material have been injected into said mold cavity and until athird predetermined number of increments of plastic material have beeninjected into said mold cavity; (m) decreasing the rate at which saidplastic material is injected into said mold cavity to maintain saidunbroken melt front after said third predetermined number of incrementsof plastic material have been injected into said mold cavity; (o) whilemaintaining said unbroken melt front, injecting said plastic materialinto said mold cavity using a pressure-dominated control algorithm aftersaid third predetermined number of increments of plastic material havebeen injected into said mold cavity and until said mold cavity is filledwith said plastic material; (p) allowing said plastic material to atleast partially harden within said mold cavity; and (q) removing saidplastic material from said mold cavity.
 18. The method of claim 17,wherein said increments of plastic material are less than about twograms.
 19. The method of claim 17, wherein said first predeterminednumber of increments of plastic material equals about ten percent ofsaid increments of plastic material.
 20. The method of claim 17, whereinsaid second predetermined number of increments of plastic materialequals about fifteen percent of said increments of plastic material. 21.The method of claim 17, wherein said second predetermined increments ofplastic material equals about seventy percent of said increments ofplastic material.